(1) Field of the Disclosure
This disclosure relates generally to amplifiers and relates more specifically to differential audio amplifiers.
(2) Description of the Prior Art
Portable (battery driven) devices usually suffer from limited acoustical loudness especially, when driving embedded mini-speaker. Besides the fact, that such mini-speakers offer a low sensitivity the electrical power capability of the (battery driven) speaker amplifier is one of the most critical factors.
In case the available speaker loudness is limited by the output power of the amplifier below current practice is known:
1.1 Increasing the Electrical Output Power by Lowering Speaker Impedance (e.g. From 8 to 4 O):
The maximum voltage swing stays the same but drives a higher current into the speaker. At the same time the ratio of speaker to connection track impedance is reduced. To stay efficient this requires routing and traces with very low resistance from the battery to the amplifier and from the amplifier towards speaker. On top 4 O speakers typically offer a lower sensitivity as similar to 8 O types (acoustical loudness versus electrical input power).
1.2 Increasing the Amplifier Efficiency by Using Switching Technology:
In the market of portable applications the dominant speaker amplifier type is Class AB. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the “class A” region, where it is amplified with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small. The crossover distortion can be reduced further by using negative feedback. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well. Class AB amplifiers are also favored in battery-operated devices. These amplifiers do not emit any electro-magnetic interference (EMI) noise (from e.g. switching) and by that are compatible with noise sensitive components like RF transceivers. Class AB amplifiers can be configured to provide a good linearity. For a constant sine wave at maximum power they may achieve efficiency in the range of >63%, but in case of music/voice (with a natural crest factor of 15 dB) the average efficiency can drop down to 20%. The relative efficiency reduces even further in case the maximum signal swing is small in comparison to the supply voltage of the output stage.
The amplifier efficiency can be improved by using switching amplifier technology like e.g. Class D. The loss of the driver stage is manly minimized by either switching it OFF or completely ON from a modulated high frequency pulse density modulation (PDM) or pulse width modulation (PWM) signal. Class D amplifiers use switching to achieve very high power efficiency. By allowing each output device to be either fully on or off, losses are minimized. In case the analog output is created by pulse width modulation; the active element is switched on for shorter or longer intervals instead of modifying its resistance. In case of PDM the analog output is created by a higher or lower density of high pulses of a fixed duration. There are more complicated switching schemes like sigma-delta modulation, to improve some performance aspects like lower distortions or better efficiency.
The efficiency of a similar rated Class D amplifier may achieve 86% for tones at maximum amplitude, which typically stays >70% even for music and voice. The impact on maximum loudness in comparison to class AB type amplifier is limited, as only the difference in maximum efficiency will be gained (15-20%).
The output signal is created out of a PWM/PDM stream either using a cascaded LC filter or the parasitic inductance of the speaker coil itself. The first solution typically requires bulky and expensive discrete components, reduces the efficiency and introduces distortion to the audio signal. The second approach requires a routing of switched high current signals between amplifier and speaker. To limit in the latter case the cross talk towards other components enforces a shielded routing, which is complicated and not possible under all conditions. As a result both solutions have obvious disadvantages and by that Class D amplifiers so far have proven a limited market acceptance especially for noise sensitive applications like mobile phones, etc.
1.3 Running the Speaker Amplifier from a Fixed Boosted Supply Voltage:
Instead of increasing the electrical current into the speaker (see 1.1) another way towards higher amplifier output power is an increased voltage swing. But any amplitude above battery voltage (VBAT) requires a boosted amplifier supply voltage. As capacitive switching regulators typically are not strong enough to drive speaker amplifiers, inductive switching boost converters are usually implemented. In case of supplying a linear amplifier the overall efficiency is reduced by the loss of the boost converter, which can be dominant for low output amplitudes. This gets worse as higher the supply voltage has to be boosted. The boost efficiency in case of light load may be improved by discontinuous switching, but emitted noise signals will no longer have a well-defined frequency, which is unwanted for noise sensitive applications.
1.4 Class H/G Amplifiers with Boosted Supply Voltage:
To improve the efficiency of a linear amplifier its supply voltage can be regulated towards a minimum voltage drop over the pass devices of the output stages. Class G amplifiers switch between discrete supply voltage levels derived either from amplifier volume settings or the average signal level. For low power headphone amplifiers this is meanwhile state-of-art by using capacitive charge pumps allowing boosting of the amplifier supply voltage as well as supply voltage division. Capacitive charge pumps of reasonable size don't provide enough current for speaker drivers that mainly use inductive DC/DC regulators. The Class H amplifier regulates its supply voltage similar, but “step-less” and by that follows the envelope of the signal level with minimum margin. In case of boosted class G or H with regulation over the whole output amplitude range the DC/DC converter has to be a buck-boost converter. In case of a true buck-boost the drawback is relative high cost (larger die size than pure buck or pure boost converter), irregular switching pattern when input and output voltages are similar and increased loss of efficiency from two serial switches required inside the buck-boost regulator.
Inside portable applications of today one can find all above described solutions (partially in combination) to overcome the maximum loudness of playback via embedded mini-speakers.
It is a challenge for engineers designing speaker amplifiers to improve speaker loudness limited by the output power of the amplifier.
There are following solutions known in the field of speaker amplifiers:
U.S. Patent Publication (US 2009/0022339 to Sasaki) discloses an amplifier including an amplifying block for receiving an input signal to drive a speaker, and a power supply block for supplying an output voltage to the amplifying block. The amplifying block includes a PWM-signal generation circuit which converts the input signal into a PWM signal, a speaker driving circuit which drives the speaker based on the PWM signal, and a duty detection circuit which outputs a time difference signal proportional to a difference between the pulse width and the pulse interval of the PWM signal. The power supply block controls the output voltage thereof based on the time difference signal. If the volume of a speaker is loud, the amplifier increases the drive voltage of the speaker to suppress distortion of the volume.
U.S. Patent (U.S. Pat. No. 7,358,814 to Seaberg) discloses a differential audio amplifier including a differential input stage for producing an output voltage in response to a differential audio input signal. The differential input stage has a first bias voltage and a second basis voltage. A bias compensation module controls the first bias voltage to be substantially equal to the second bias voltage. An objective of the disclosure is to achieve an audio amplifier providing greater output voltage swings while maintaining low total harmonic distortion (THD).
U.S. Patent (U.S. Pat. No. 6,792,121 to Koyama et al.) proposes an audio signal amplifier circuit including an external output terminal of an IC to which an output line of a power amplifier circuit is connected, a first resistor connected between a certain terminal of the IC other than the external output terminal and a feedback input of a differential amplifier circuit, a first capacitor connected between the external output terminal and a loud speaker, a second capacitor between the certain terminal of the IC and a wiring line between the external output terminal and the loud speaker, a filter circuit provided on a signal input side of the differential amplifier circuit and including a second resistor and a third capacitor for attenuating signal components having frequencies in a middle and high frequency ranges and voltage follower means provided between an input stage and an output stage of the audio signal amplifier circuit, wherein the first capacitor is a small capacitor having a capacitance value in the order of 30 μF or smaller and an attenuation characteristics of the first capacitor and an impedance of the loud speaker in a low frequency range is compensated for by a relative amount of increase of amplification gain in the low frequency range determined by a time constant of the second capacitor and the first resistor and a time constant of the third capacitor and the second resistor.